Metal Ions Induce Growth and Magnetism Alternation of α-Fe2O3 Crystals Bound by High-Index Facets

• Published in: Chemistry : a European journal 2012-07, Vol.18 (29), p.8957-8963
• Main Authors: Liu, Rongmei, Jiang, Yuanwen, Fan, Hao, Lu, Qingyi, Du, Wei, Gao, Feng
• Format: Article
• Language: English
• Subjects: controlled synthesis; crystal engineering; high-index facet; magnetic properties; α-Fe2O3
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201201108

In this study, quasi‐cubic and hexagonal bipyramid α‐Fe2O3 polyhedrons with high‐index facets exposed were controllably synthesized by applying metal ions Zn2+ or Cu2+ as structure‐directing agents. The growth of the α‐Fe2O3 nanostructures with high‐index facets were induced by metal ions without the addition of any other surfactants. The quasi‐cubic form controlled by Zn2+ looks like a cube but has an angle of approximately 86° bound by (012), (10‐2), and (1‐12) facets, whereas the hexagonal bipyramid form controlled by Cu2+ has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape‐ and surface‐dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin‐canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi‐cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal‐ion‐directing technique could provide a universal method for shape‐ and surface‐controlled synthesis of nanocrystals with high‐index facets exposed. Ioned into shape: Two kinds of α‐Fe2O3 polyhedrons, quasi‐cube and hexagonal bipyramid (see SEM images), bound by high‐index facets were controllably synthesized by applying metal ions Zn2+ or Cu2+ as structure‐directing agents. The crystals display different magnetic properties.